Non-volatile memory is memory that can continue to store data after power is no longer provided to the memory. “Flash memory,” called this because data can be erased from multiple memory cells simultaneously, is an example of non-volatile memory. A typical flash memory comprises an array of memory cells having the cells arranged in rows and columns of memory. The array is broken down into blocks of memory cells. Although each of the cells within a block can be electrically programmed to store data individually, data is erased from the cells at the block level.
A common example of flash memory is NAND flash memory. The array of memory cells for NAND flash memory devices are arranged such that a control gate of each memory cell of a row of the array is connected to a word line. However, each memory cell is not directly connected to a column bit line. Instead, the memory cells of the array are arranged together in strings (“NAND strings”), with the memory cells connected together in series, source to drain, between a source line and a column bit line. The NAND strings can have as many as 32 memory cells between the source line and the column bit line.
The memory array for NAND flash memory devices is then accessed by a row decoder activating a row of memory cells by selecting the word line connecting the control gates of the memory cells. In addition, the word lines connected to the control gates of unselected memory cells of each string are driven so that the respective memory cell passes current in a manner that is unrestricted by their stored data values. Current then flows from the source line to the column bit line through each series connected string, restricted only by the selected memory cells of each string. This places the current-encoded data values of the row of selected memory cells on the column bit lines to be sensed and amplified before being output as data, as well known.
In an effort to provide greater memory density in a relatively small package, multiple NAND flash memories are packaged together as one device to provide a multi-chip NAND flash memory device. In some arrangements, the NAND flash memories share common circuits and busses so that from all outward appearances, the multiple NAND flash memories are controlled and operated as a single, larger capacity NAND flash memory device. As a result, memory commands issued to the multi-chip memory device may be executed by one or some of the NAND flash memories, while other memory commands issued to the multi-chip memory are executed by all of the NAND flash memories. The latter type of memory commands are often referred to as global memory commands, which cause common operation of all or a substantial number of the discrete NAND flash memories at one time.
An example of a global memory command is a reset command, which can be used to abort a command sequence in progress and put the individual NAND flash memories in a known condition. Thus, when a global reset command is issued to a multi-chip NAND flash memory device, all of the individual NAND flash memories in the multi-chip memory respond by executing the reset command.
A result of issuing a global memory command to a multi-chip device is that there is a sudden increase in its power consumption due to multiple devices beginning execution on the memory command. For example, the global memory command may require that on-board charge pumps be activated and provide elevated voltages for carrying out the command. The sudden increase places a significant load on the power supply providing power to the multi-chip device. In cases where the power supply has insufficient capacity, the voltage and current may dip, affecting not only the performance of the multi-chip device, but other electrical devices that rely on the same power supply. In battery powered applications, sudden increases in power consumption are undesirable because of the power supply's limited availability to handle peak power demands as well as limited overall availability of power.
Therefore, there is a need for a system and method that mitigates the peak power demand that occurs in response to issuing a global command to a multi-chip device.